The present invention relates generally to two-speed transfer cases for use in four-wheel drive vehicles. More particularly, the present invention relates to a control system for shifting the transfer case on-the-move between high-range and low-range drive modes.
The drivetrain in many light-duty and sport-utility vehicles includes a transfer case for transmitting drive torque from the engine and transmission to all four of the wheels, thereby establishing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many transfer cases are equipped with a gear reduction unit which can be selectively shifted by the vehicle operator to establish four-wheel high-range and low-range drive modes. In many instances, however, the transfer case can not be shifted between its four-wheel high-range and low-range drive modes unless the vehicle is stopped and the transmission is in neutral. Unfortunately, the need to stop the vehicle prior to such "range" shifting is inconvenient, particularly upon encountering road conditions or surface terrains where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered.
To alleviate this inconvenience, some gear reduction units are equipped with synchronizer clutches which permit the vehicle operator to shift from the four-wheel low-range drive mode into the four-wheel high-range drive mode without stopping the vehicle. For example, U.S. Pat. No. 5,054,335 discloses a transfer case equipped with a synchronized range shift arrangement for on-the-move shifting of a layshaft-type gear reduction unit. Alternatively, commonly-owned U.S. Pat. No. 5,346,442 discloses a transfer case having a synchronized range shift arrangement for on-the-move shifting of a planetary-type gear reduction unit. In addition, commonly-owned U.S. Pat. No. 5,655,986 discloses a transfer case equipped with a planetary-type gear reduction unit which permits synchronized shifting into and out of the high-range drive mode and low-range drive mode.
In addition to the above-noted mechanically synchronized range shift system, it is also known to electronically control the drivetrain to provide on-the-move range shifting of transfer cases. For example, an electronically-shifted two-speed transfer case is disclosed in U.S. Pat. No. 5,522,777 as having a transfer case control module which receives input signals from the engine control module relating to the current transmission gear, engine speed and vehicle speed. The transfer case control module uses these input signals to generate control signals that are sent back to the engine control module for use in regulating vehicle operation to accommodate on-the-move range shifting. Specifically, the engine control module will control the engine fuel system to modify engine speed and/or shift the transmission gear to match the rotary speed of the transmission output to that of the transfer case output prior to actuation of the range shift mechanism. However, the requirement to integrate such a transfer case control system into the existing circuitry of conventional engine control modules increases the cost and complexity of the four-wheel drive system.
In view of the substantial interest in transfer cases equipped with on-the-move range shift systems, a recognized need exists to develop electronically-controlled range shift systems for use with four-wheel drive transfer cases that further advance the current technology.